A new study released by the United Nations Children's Fund (UNICEF) indicates the West African nation of Sierra Leone has the world's highest rate of child mortality.

The study, released early Friday says one in four children in Sierra Leone dies before the age of five.

UNICEF has been tracking child mortality rates worldwide since 1990 and has established a goal of reducing them by two-thirds by the year 2015.

The agency says recent data indicate that child mortality rates in some areas, such as sub-Saharan Africa, have actually gotten worse since 1990. It cites the high incidence HIV-AIDS as the chief cause.

Armed conflicts in have also contributed to high child mortality rates in Iraq and Afghanistan.

According to UNICEF, Malaysia, Malta and Egypt have made the most dramatic improvements in child mortality rates among individual nations.

The study also shows that Cuba and Latin America as a whole have shown the most substantial improvement on average.